can't disconnect/destroy zfs pool

I try to destroy a pool on WebUI and get folowing error:

Error: concurrent.futures.process._RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/middlewared/plugins/zfs.py", line 139, in do_delete
    zfs.destroy(name, force=options['force'])
  File "libzfs.pyx", line 382, in libzfs.ZFS.__exit__
  File "/usr/lib/python3/dist-packages/middlewared/plugins/zfs.py", line 139, in do_delete
    zfs.destroy(name, force=options['force'])
  File "libzfs.pyx", line 1286, in libzfs.ZFS.destroy
libzfs.ZFSException: umount failed

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/lib/python3.8/concurrent/futures/process.py", line 239, in _process_worker
    r = call_item.fn(*call_item.args, **call_item.kwargs)
  File "/usr/lib/python3/dist-packages/middlewared/worker.py", line 91, in main_worker
    res = MIDDLEWARE._run(*call_args)
  File "/usr/lib/python3/dist-packages/middlewared/worker.py", line 45, in _run
    return self._call(name, serviceobj, methodobj, args, job=job)
  File "/usr/lib/python3/dist-packages/middlewared/worker.py", line 39, in _call
    return methodobj(*params)
  File "/usr/lib/python3/dist-packages/middlewared/worker.py", line 39, in _call
    return methodobj(*params)
  File "/usr/lib/python3/dist-packages/middlewared/schema.py", line 977, in nf
    return f(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/middlewared/plugins/zfs.py", line 144, in do_delete
    raise CallError(str(e), errno_)
middlewared.service_exception.CallError: [EBUSY] umount failed
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/middlewared/job.py", line 372, in run
    await self.future
  File "/usr/lib/python3/dist-packages/middlewared/job.py", line 408, in __run_body
    rv = await self.method(*([self] + args))
  File "/usr/lib/python3/dist-packages/middlewared/schema.py", line 973, in nf
    return await f(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/middlewared/plugins/pool.py", line 1521, in export
    await self.middleware.call('zfs.pool.delete', pool['name'])
  File "/usr/lib/python3/dist-packages/middlewared/main.py", line 1238, in call
    return await self._call(
  File "/usr/lib/python3/dist-packages/middlewared/main.py", line 1196, in _call
    return await methodobj(*prepared_call.args)
  File "/usr/lib/python3/dist-packages/middlewared/service.py", line 475, in delete
    rv = await self.middleware._call(
  File "/usr/lib/python3/dist-packages/middlewared/main.py", line 1204, in _call
    return await self._call_worker(name, *prepared_call.args)
  File "/usr/lib/python3/dist-packages/middlewared/main.py", line 1210, in _call_worker
    return await self.run_in_proc(main_worker, name, args, job)
  File "/usr/lib/python3/dist-packages/middlewared/main.py", line 1137, in run_in_proc
    return await self.run_in_executor(self.__procpool, method, *args, **kwargs)
  File "/usr/lib/python3/dist-packages/middlewared/main.py", line 1111, in run_in_executor
    return await loop.run_in_executor(pool, functools.partial(method, *args, **kwargs))
middlewared.service_exception.CallError: [EBUSY] umount failed

Steps to reproduce:

Storage

Export/disconnect pool:

Error:

in cli also did not work: